1. Field of the Invention
The invention relates to a semibatchwise process for the mild distillative separation of mixtures.
2. Brief Description of the Prior Art
Continuous separation processes using a column and batchwise separation processes by means of a distillation bulb with attached column are known. Said processes are described in the form of a synopsis, for example in “Thermische Trennverfahren” [Thermal separation processes] (K. Sattler, 2001 edition, VCH Verlagsgesellschaft, Weinheim).
In batchwise separation processes, which are frequently used for the separation of two, in particular more than two components, a distillation bulb is first filled with the mixture to be separated and heated to the boiling point. The fractions can then be taken off in ascending order of their boiling point via the top of the column, with the bottom temperature simultaneously increasing. This procedure proves to be disadvantageous particularly when, because of its boiling point, a component is not taken off as the first fraction and tends to undergo secondary reaction, such as, for example, decomposition, under prolonged thermal load.
In the case of demanding requirements with respect to the purity of these higher-boiling components, it is possible to take off intermediate cuts of lower purity. These are then added to the feed, for example in the next separation. Consequently, a part of the feed is circulated, with the result that the space-time yields decrease considerably. A further disadvantage of the batchwise procedure is the substantially higher specific energy demand in comparison with the continuous procedure (cf. Kaibel, Krug, Chemie Ingenieur Technik [Chemical Engineering] 70, 1998, pages 711-713).
German Offenlegungsschrift 42 26 905 describes a batchwise distillation, with a part-fraction being taken off in a side stream and being recycled into the columns later on. In this way, the specific energy demand is slightly reduced. However, a disadvantage thereof is that the process control is substantially more complicated and an additional container is required for intermediate storage. Furthermore, in this process, the bottom container is initially filled and higher-boiling components are subjected to thermal load during the entire distillation time in the boiling liquid bottom product.
EP-A 638 345 describes a column having a container at the bottom and at the top, which can be used as a rectification or stripping section depending on the desired procedure. For mixtures whose medium boiler fraction accounts for a proportion of at least 70% by weight, the distillation time can be reduced by this procedure. However, this process requires complicated apparatus and cannot generally be used.
Baichun and Zhicai (Huagong Gongcheng, 22, 1994, pages 30-34) describe the separation of a multicomponent system in a column having two containers. A complete non-steady-state process of a batchwise mode of operation is divided into n−1 continuous steps in order to separate the mixture having n fractions. A container is required as a feed container and a second one is required in order to receive the bottom discharge. This procedure permits the simultaneous use of the column as a stripping and rectification section and hence an acceptable specific energy consumption. However, an additional container is required for this process too.
Maczynski and Maczynska (Przemysl Chemiczny 49, 1970, pages 599-601) propose, for the separation of a binary mixture, a process which combines the advantages of the continuous procedure and the batchwise procedure. In a first step, the column at whose lower end a distillation bulb is present is supplied with a continuous feed and the first fraction is taken off via the top of the column. The bottom discharge is used for filling the distillation bulb. In this process too, the content of the distillation bulb must be kept at the boiling point, and the thermal loading of the higher-boiling components is therefore very high.
There is therefore the need to provide a process for the distillative separation of mixtures which has an advantageous specific energy demand, keeps the complexity of the apparatus manageable and keeps the thermal loading of sensitive components small. Furthermore, an improvement for mixtures of any desired composition should be achievable.